Method of measuring the velocity and/or length of endless webs of textile material and apparatus for carrying out the method

ABSTRACT

A method and an apparatus for measuring the velocity and/or length of endless webs of textile goods are described. The textile web is irradiated with a light beam of which one deflects a first portion prior to impinging on the surface of the textile web and one directs the deflected light beam to evaluation means. The light beams reflected from the surface of the textile web are directed through a convex lens and are also directed to the evaluation means so that the corresponding light beams are superimposed. The velocity and/or the length of the web of textile goods are determined from the frequency of the interference formed by the superposition of the light beams.

The present invention concerns a method of measuring the velocity and/orlength of endless webs of textile material and an apparatus for carryingout this method.

Normally, in the textile industry the velocities or lengths of conveyedendless webs are measured according to the mechanic rolling method.According to this method a measuring wheel which is connected to aspeedometer is brought into contact with the respective web which has tobe measured. This web is for instance a two-dimensional structure or ayarn. Furthermore, it is known to connect the measuring wheel to adeflection roller at which the endless web is deflected in order todetermine in such a manner the velocity of the transported web which isin contact with the deflection roller by measuring the velocity or thenumber of revolutions of the deflection roller.

However, the above-described mechanic rolling method can cause a numberof problems. So, for instance, with sensitive webs the danger existsthat the surface of the web is damaged by the positioning of themeasuring wheel which, for instance, results in a surface rougheninglinearly extending in the running direction of the web or in displacedspots. The application of the measuring wheel can result in adeformation in this area especially with elastic articles, as forinstance knitted fabrics, which brings along corresponding faultymeasurements of the length of the web. The above-described second methodaccording to which the web velocity or the web length is derived fromthe velocity of the deflection roller or its number of revolutionsbrings along the danger of faulty measurements, too. So, for instance,by this slippage effects between the web and the deflection roller whichsubstantially influence the results of the measurements cannot beexcluded. This is especially then the case if wet webs are conveyed oversuch a deflection roller or if the web makes necessary an especiallysmooth and polished deflection roller due to its sensitivity with regardto mechanical stresses.

It is the object of the invention to provide a method and an apparatusof the cited kind with which an especially exact measurement of thevelocity and/or the length of running webs is possible.

This object is achieved by a method having the characterizing featuresof the claims and by an apparatus having the characterizing features ofthe claims.

The inventive method is based on the fundamental idea to not carry outthe measurement of the velocity or of the length of the running web bydirect contact of a measuring wheel with the web or by deflecting theweb over a deflection roller the velocity or number of revolutions ofwhich is determined, as this is the case with the above-cited prior art.The inventive method rather proposes a direct and contact-freemeasurement of the velocity or of the length of the web. According tothis method, a light beam is directed to the running web wherein aportion of the light beam has been deflected before and has beensupplied to evaluation means. Surpisingly, it could be observed that thebeam impinging on the surface of the textile web is reflected by thesurface such that a reflection area results which is still relativelynarrow. This is surprising since one would have expected that the webwould reflect in all directions, scatter and/or adsorb the directedlight beam on account of the plurality of reflection areas of such asurface and of the adsorption characteristics of the web so that areflection area would result which cannot be exactly evaluated. Asalready stated above, this does not result with the inventive method sothat a selected portion of the light beams from the reflection area canbe directed through a convex lens and can be superimposed with thatportion of the light beam which has been deflected before. By this,various Doppler frequencies, i.e. interferences, result in response tothe respective velocity of the web which have the result of intensityvariations of the light beam. Evaluation means which receive thesuperimposed light beams determine the velocity of the web from theinterferences according to the following formula: ##EQU1## wherein thefactors have the following meaning: f the determined interference,

v the velocity of the web,

α the irradiation angle and

λ the wavelength of the light beam.

From the above equation the following equation results for the weblength 1 transported in the time unit t: ##EQU2## wherein n is thenumber of the counted Doppler oscillations within the time t.

With the inventive method a single light beam or preferably a bundle oflight beams of the same wavelength can be used. Accordingly, the usedterm "light beam" covers not only a single light beam but also acorresponding bundle of monochromatic light beams.

The above-described inventive method has the advantage that itguarantees a very exact measurement of the velocity and/or length of theweb since the propagation velocity of the light beam relative to thevelocity of the web is higher by about a plurality of powers of ten sothat no measuring errors can result by this. So, for instance, it couldbe observed that the measuring error could be significantly reducedcompared with the known methods, i.e. it was only 1/100 to 1/1000 of theoriginal measuring error. Furthermore, with the inventive method theabove-described problems which result from the use of a measuring wheelwhich is in contact with the web no longer exist since, due to thecontact-free measurement, no undesired displacements of the threadsystems, roughenings or other mechanical damages of the surface of theweb can take place. The inventive method measures uniformly andreproducibly the velocity or the length of the web independetly of therespective condition thereof, i.e. independently of the fact whether theweb is dry or wet. Furthermore, the method can be used especially wellfor controlling the velocity of the web, due to its high reproducibilityand exactness, as is explained in detail in connection with an exampleof the apparatus.

On principle, every light beam can be used with the inventive methodprovided the light beam is monochromated prior to the superposition ofthe deflected and reflected light beam. However, it is easier to use alight source which generates a monochromatic light beam. Especially goodresults are achieved if one uses a laser beam or, as already mentionedabove, a bundle of laser beams. Here, it is no more necessary tomonochromate the light beam since the laser automatically generateslight beams of only one wavelength. For this the known lasers can beused as light source, as for instance inert gas lasers, He-Ne-lasers orCo₂ lasers, as is explained in connection with the apparatus.

As regards the irradiation angle, it has to be noted that the same canvary between about 20° and 89.5° or about 160° and 90.5°, preferably ina range of between about 80.5° or about 100° and 90.5°, relative to therunning direction of the web. The preceding preferred irradiation anglerange (slightly less than 90° or slightly larger than 90°) allows theuse of relative small convex lenses since with increasing or decreasingsize of the irradiation angle, i.e. with angles of between about 20° andabout 70° or 160° and 110°, correspondingly large convex lenses arenecessary which cause higher costs and enlarge the apparatus in itsvolume. Especially small lenses for the concentration of the reflectedlight beam can be used if the surface of the web is irradiated withirradiation angles of between 89.5° and 89.9° or 90.1° and 90.5°. Theseirradiation angles are to be used especially for cases when very highweb velocities are to be measured. In this connection it has to be notedthat the inventive method cannot be used with irradiation angles of 90°since with such an irradiation angle no frequency difference between thedeflected and the reflected light beam and thus no intensity variationresults due to the missing velocity component in the direction ofirradiation.

Especially good results can be achieved if one selects irradiationangles of 86.5° or 93.5° relative to the running direction of the web.So, it was observed that with such an irradiation angle the measuringresult is not influenced or is influenced only in a negligible manner byflutter movements of the web perpendicularly with respect to the runningdirection. This is important since such flutter movements are especiallycaused if the web is transported with a relatively high velocity, i.e.with velocities of between about 20 m/min and about 80 m/min, or overgreater distances without any additional support, as this is frequentlythe case with production machines. In such cases one can achieve veryexact and reproducible measuring results if the measurement is carriedout at locations at which the web is supported by a deflection rollerand the web is transported not only in web running direction but alsosimultaneously perpendicularly to the running direction due to anunbalance of the deflection roller. Thus, it has to be noted that withan irradiation angle of 86.5° or 93.5° relative to the running directionof the web the above-described flutter movements or movements of the webperpendicularly with respect to the running direction of the web can beeliminated in such a manner that they have no influence or have only anegligible influence on the measurement of the length or velocity of theweb.

With respect to the intensity, energy or capacity of the used lightbeam, it has to be stated that limits are predetermined by the substrateand the processing condition of the web since no undesired modificationsof the material, as for instance fusing of the web surface ordestruction of the dyes, are to result on account of the measuringmethod. So, light beams having a capacity density of about 4 mW/cm² upto about 15 mW/cm² have had an especially advantageous effect sinceherewith no undesired modifications of the above-cited kind are caused.

In order to achieve an especially reproducible measurement free ofundesired noise effects, a further embodiment of the inventive methodresides in adapting the intensity of the deflected light beam to theintensity of the light beam reflected from the web surface of thetextile material prior to its feeding to the evaluation means. Thisresults in the superposition of light beams (deflected and reflectedlight beam) having approximately the same intensity so that thefrequency of the interference caused by this superposition and thus alsothe respective velocity or length of the web can be determinedespecially exactly.

Such an intensity adaption of the deflected light beam to the intensityof the light beam reflected from the web surface can be achieved bypartly reflecting the deflected light beam at suitable reflection meansand/or partly absorbing the same prior to its feeding to the evaluationmeans so that only the reflected and/or non-absorbed portion of thedeflected light beam is superimposed with the light beam reflected fromthe web surface, as this is still explained in detail in connection withthe apparatus. Such a method enables also the measurement of velocitiesor lengths of webs of such materials which, for instance due to itscolouring, absorb a large portion of the light beam impinging on the websurface or which, on account of their micro structure or macrostructure, i.e. their roughness, have a relatively broad reflection areawhich can be evaluated only with difficulty.

The inventive apparatus for carrying out the above-described methodcomprises a light source serving for the generation of the light beam. Abeam dividing means is located between the light source and the web,said beam dividing means deflecting from the light beam a first portionwhich is used as reference beam in the above-described method. This beamdividing means is known to the expert in the art and, for instance,deflects the reference beam necessary for the measuring method by meansof a prism and lets pass a second portion of the light beam through thebeam dividing means without being influenced. A convex lens is locatedbetween the beam dividing means and the web, said convex lensconcentrating selected reflected light beams from the lobe likereflection area. Furthermore, evaluation means are provided comprising aphoto detector and signal processing means. The selected andconcentrated reflected beams superimposed by the reference beam aredirected to the photo detector so that the same transforms the Dopplerfrequency of the interference or the intensity variations of the lightbeam superimposed by the reference beam into an electric signal.

Preferably, the light source is a laser on account of the above-citedreasons. Especially, the laser is an inert gas laser, particularly aHe-Ne-laser having a light capacity of between about 2 mW and about 8 mWwhich irradiates laser beams having a wavelength of 632.8 nm. Such aninert gas laser can also be replaced by a Co₂ laser or solid-state laserhaving a corresponding capacity and known to the expert in the artwherein these lasers generate light beams as continuous beams.

In an especially suited embodiment the photo detector has a measuringarea of <1 mm², preferably of about 0.2 mm² up to about 0.5 mm². Suchphoto detectors, especially such having a measuring area of about 0.2mm², enable an optimization of quantum yields compared with diodeshaving larger measuring areas so that the same are larger than about 50%at wavelengths of about 600 nm while diodes having measuring areas >1mm² enable quantum yields of only about 20%. By this, it is assured thatalso light beams reflected from the web surface with relatively lowintensity can be evaluated without any difficulties wherein there is nodanger that the measurement is disturbed by the noise caused by thesignal processing means, especially in the low frequency range ofbetween about 500 Hz and about 1000 kHz. This has the result thatDoppler signals with little noise can be evaluated at high signalamplification.

Especially good results can be achieved with an inventive apparatuswhich includes as photo detector either a p-i-n-type diode or a siliconeavalanche diode. Especially the silicone avalanche photo diode has anessentially better quantum yield and a higher resolution in thewavelength range of between about 500 nm and about 900 nm while thep-i-n-type diode is better than the silicone avalanche photo diode inthe range of between about 900 nm and about 1000 nm, as this can betaken from the comparison in FIG. 6 which is described in the following.An especially high frequency resolution can be achieved with barrierlayer photo detectors which function in a frequency range of betweenabout 500 Hz and about 1000 kHz, wherein a frequency range around about500 Hz is especially advantageous for the inventive method or theinventive apparatus. On account of the tuning of the blocking voltageand on account of the inner amplification the signal-noise-ratio can beadapted to the respective application, i.e. to the macro structureand/or micro structure of the web (roughness) and/or colour.

Furthermore, it has to be stated that the design of the convex lenswhich is located between the beam dividing means and the web has anessential influence on the exactness of the measurement. Especiallysuited are convex lenses the focus of which lies between about 40 mm andabout 60 mm, preferably at about 50 mm, and the diameter of which isless than 25 mm, preferably at about 19 mm. With such a convex lens alsothe velocity or length of relatively narrow webs can be detected.

An especially suitable embodiment of the inventive apparatus comprisesreflection means which are located in the beam path of the deflectedlight beam prior to the evaluation means. The reflection means has theeffect that the intensity of the deflected light beam can be adapted tothe intensity of the light beam reflected from the web surface such thatonly a portion of the deflected light beam is superimposed with thereflected light beam for generating the interference which has to beevaluated for the measurement. In the simplest case this reflectionmeans consists of a glass pane which is located in the beam path of thedeflected light beam so that it reflects only a portion of the deflectedlight beam to the photo detector. If an apparatus is used according towhich the beam dividing means is formed as beam dividing tube, aninterface of the beam dividing tube can serve as reflection means.During the transition of the deflected light beam from the glass intothe air a reflected part beam develops which is superimposed as areference beam with the light beam reflected from the web surface.

Furthermore, there is the possibility to provide an absorption meansinstead of the reflection means or in addition to the reflection meanswherein in the simplest case the interface described before is providedwith an absorption filter. This absorption means has the effect thatonly a portion of the deflected light beam is fed to the measurement andprevents at the same time through absorption that the non-reflectedportion of the deflected light beam falsifies the measurement result. Onprinciple, any material which adsorbs the portion of the deflected lightbeam that passes through the interface is suited for such an adsorptionfilter located on the interface of the beam dividing cube. Especiallygood results can be achieved with an adsorption filter consisting ofblack velours and located from outside on the corresponding interface.

According to another embodiment of the inventive apparatus theadsorption means is formed as a linear circular transmission graduatedfilter. Such a filter allows the adaptation of the intensity of thedeflected light beam to the intensity of the light beam reflected fromthe web surface in an especially simple manner wherein the intensity canbe adjusted in a range of between 0% and 100%. By this, in an especiallysimple manner the inventive apparatus can be adapted to the macrostructure and micro structure and colour of the web which has to bemeasured.

In the inventive apparatus the signal processing means is directlycoupled to the photo detector. This signal processing means, in thesimplest case, consists of an oscilloscope which displays theinterference generated by the superposition of the light beams and/orcounts the Doppler frequency. A counting of the Doppler frequency(interference frequency) is necessary for the quantitative evaluation ofthe velocity or length of the web in accordance with the above-mentionedformulas. However, if one wishes to determine the velocity of the web ata plurality of locations with the inventive method which, for instance,is especially advantageous for the tuning of various drive motors in amachine, or if one wishes to tune the velocity of several webs, it issufficient in the simplest case to display the respective interferenceat the corresponding measurement location or the respective interferenceof the corresponding web on the oscilloscope and to compare it with apredetermined interference and to manually correct the correspondingdrive motor of the web if a deviation occurs. Of course, such acorrection can be carried out automatically. Here, the signal processingmeans generates an output signal which is used for the control of thespeed of the drive motors for other web locations or the drive motorsfor the other webs if a predetermined value is fallen short of or isexceeded, said value, for instance, is predetermined by the web velocityat a selected location or for the case of the determination of severalweb velocities by the velocity of a selected web.

According to the above-described application of the inventive method forthe control of the velocity of a web at a plurality of locations or fortuning the velocity of a plurality of webs, an embodiment of theinventive apparatus is used having a corresponding number of lightsources, beam dividing means, convex lenses as well as evaluation meansdependent on the locations which have to be controlled with one web ordependent on the number of webs if the velocities of a plurality of websare to be synchronized. Here, the evaluation means comprise a number ofphoto detectors and signal evaluation circuits corresponding to thelocations or webs. If a manual tuning of the velocities is carried out,the signal processing circuits can consist of an oscilloscope having acorresponding number of inputs so that the signals generated by thephoto detectors can be displayed on the screen of the oscilloscope andthereafter the drive motors can be independently controlled. Accordingto such an automatic control the signal processing circuits generate anumber of output signals corresponding to the number of measuringlocations which are used for the control of the speed of thecorresponding drive motors, respectively.

An especially suited embodiment of the inventive apparatus which isespecially suited for an industrial measurement is formed as compactunit. This embodiment has an elongated housing which is preferablyclosed on all sides and which has coupling means for the light source atits front-sided end. The coupling means is formed such that it isadapted to be coupled to the light source either directly or through aoptical fiber located between the light source and the housing. A lightbeam inlet and outlet aperture, preferably the above-described convexlens, is located at the front side which is opposite to the couplingmeans. Through this aperture the measuring beam leaves the housing, andthe light beam reflected from the web surface re-enters the housingthrough this aperture. The beam dividing means, possibly the reflectionmeans for the deflected light beam as well as the signal processingcircuit are located within the housing.

In order to enable a simple adjustment of the focus of the convex lenson the web surface, a further embodiment includes additionally a flangedtube at the front side opposite to the coupling means, said tube holdingthe convex lens such that it is adapted to be displaced in the directionto the web surface or oppositely. For instance, this can be achieved bydesigning the tube in a telescope-like manner or providing the same witha corresponding screw thread. The tube can also be removably connectedto the housing so that, dependent on the respective measuring problem,the tube and the convex lens firmly connected therewith can be simplyand quickly replaced by another tube and another corresponding convexlens.

In order to reduce the size of such an embodiment and to provide acompact measuring apparatus, according to a further embodiment one canprovide three plane deflection mirrors feeding the deflected light beamto the photo detector. These plane deflection mirrors are locatedparallel with respect to the light beam generated by the light source.

Furthermore, with such embodiments using a beam dividing cube, aninterface for the reflection and/or adsorption of the deflected lightbeam can be used so that two plane deflection mirrors can be renouncedwith compared with the above-described embodiment, as this is stillexplained in detail in connection with an example.

Preferred embodiments of the inventive method and of the inventiveapparatus are indicated in the dependent claims.

The inventive apparatus is described in the following in connection withthe examples and with the drawing. In the drawing:

FIG. 1 is a schematic representation of a first embodiment of theapparatus;

FIG. 2 is a schematic representation of a second embodiment of theapparatus;

FIG. 3 is a schematic representation of a third embodiment of theapparatus;

FIG. 4 is a side view of an apparatus with compact design;

FIG. 4a is a sectional view along line A-B of the embodiment shown inFIG. 4;

FIG. 4b is a top view of the embodiment according to FIG. 4;

FIG. 5 is a graph of the Doppler frequency in relation to the webvelocity;

FIG. 6 is a graph of the quantum yield in relation to the wavelength ofthe light beam for two photo detectors; and

FIG. 7 shows the principle of a further embodiment of the apparatuswhich serves for the control of the velocities of two webs.

The apparatus for measuring the velocity and/or length of a web whichhas the reference number 1 in FIG. 1 includes a light source 2 directinga monochromatic light beam or a bundle of monochromatic light beams tothe surface of a web 5. The light beam has a constant wavelength λ.According to the embodiment shown in FIG. 1 the light source 2 is aHe-Ne-laser wherein the laser beam 10 has a wavelength of 632.8 nm and alight capacity of 3 mW. A portion 10a of the light beam 10 is deflectedat a beam dividing means 3 which is formed as beam dividing cube to areflection means 8 wherein the reflections means 8 is formed asadjustable plane plate. Due to the partial transparency of the planeplate 8 the intensity of the light beam 10a can be adapted to theintensity of the light beams reflected from the web surface whereby themeasuring exactness and reproducibility of the apparatus can beimproved. The other portion 10b of the light beam is directed by aconvex lens 4 at a certain irradiation angle α to the surface of the web5 which is transported with a certain velocity v in the direction of thearrow 9. The focus of the convex lens 4 is adjusted to the surface ofthe web. From the surface of the web 5 the light beam 10b is reflectedin a lobe-like manner wherein two reflection beams 10c and 10d which areonly examplarily shown are directed through the lens 4 and are deflectedby the beam dividing means and are superimposed with the light beam 10areflected by the plane plate 8. Dependent on the web velocity v avariable Doppler frequency f_(d) develops which is detected by means ofa photo detector 6 onto which the light beams 10a, 10c and 10d aredirected and is transformed there into an electric signal which is shownon the screen as an interference by a signal processing circuit 7 whichis an oscilloscope in the embodiment shown in FIG. 1.

The second embodiment of the apparatus shown in FIG. 2 comprises, asalready explained in connection with the embodiment of the apparatus ofFIG. 1, a light source 2 generating a light beam 10 having a constantwavelength λ wherein this light source is also a laser. A portion of thelight beam 10a is deflected to a reflection means 8 by a beam dividingmeans. The reflection means 8 is formed as adjustable plane plate. Thereflected portion of the light beam 10a is reflected to the firstreflection means 8 by a second reflection means 8a which is formed asadjustable plane deflection mirror and is deflected therefrom to thephoto detector 6 by means of a third reflection means 8b. The intensityof the deflected light beam 10a can be optimally adapted to theintensity of the light beam reflected from the web surface through sucha deflection and reflection of the light beam portion 10a by means ofthe adjustable reflection means 8, 8a and 8b whereby the measuringexactness of the apparatus is significantly improved. Another portion10b of the light beam 10 passes the beam dividing cube 3 and arrivesthrough a lens 4 at the surface of the web 5 to be measured which istransported with a certain velocity v which is to be measured in thedirection of the arrow 9. The irradiation angle α is in the present case86.5°. A portion of the light beam 10b is reflected in a lobe-likemanner from the surface of the web wherein two, only examplarily shownreflection beams 10c and 10d are directed through the lens 4. The web 5is located in the focus of the convex lens 4. The reflected light beams10c and 10d are deflected to the third reflection means 8b by the beamdividing cube 3 and are there superimposed with the correspondingportion of the light beam 10a. Dependent on the velocity v of the web aDoppler frequency f_(d) of different size develops which is detected bymeans of a photo detector 6 onto which the light beams 10c and 10d aswell as the part beam 10a are directed and which is transformed thereinto corresponding electric signals which are processed by the signalprocessing circuit 7 corresponding to the manner as schematically shownin FIG. 2.

The embodiment of the apparatus for measuring the velocity and/or lengthof webs as shown in FIG. 3 differs from the embodiment shown in FIG. 2by the feature that the part light beam 10a branched from the light beam10 is partially reflected at an interface A during theglass-air-transition such that a part light beam develops which has asignificantly reduced intensity which, however, has the original lightbeam frequency or laser frequency. In order to prevent an undesired backirradiation of the portion of the light beam 10a which was not reflectedat the interface A from outside of the beam dividing cube 3 into thebeam dividing cube, the interface A is provided with an adsorptionfilter C. This adsorption filter consists of a metal plate provided withblack velours. The portion of the light beam 10a reflected from theinterface A is superimposed with the light beams 10c and 10d reflectedfrom the web surface within the beam dividing cube and is fed to themeasuring area of an avalanche diode 6 which is located below aninterface b of the beam dividing cube 3 which detects the developingDoppler signal. Such an embodiment has the advantage that it is verysmall and compact. In other respects, the embodiment shown in FIG. 3 hasthe same structure and the same function as the above-describedembodiment according to FIG. 2. For clarification purposes the samereference numbers have been used for the same members.

Instead of the adsorption filter C a linear circular transmissiongraduated filter can be used either by which the intensity of theportion of the light beam 10a necessary for the measurement can beoptionally varied in a range of between 0.1% and 100%. By this it isachieved that the intensity of the deflected light beam portion 10a canbe adapted to the intensity of the reflected light beams 10c and 10ddependent on the respective measuring problem, i.e. the macro structureor the micro structure, and thus to the reflection characteristics ofthe web surface in a simple and fast manner.

Another embodiment of the inventive apparatus which is a compact unit isshown in FIGS. 4, 4a and 4b. According to this embodiment the beamdividing means 3 formed as beam dividing cube, the reflection means 8b(FIG. 4b), the adsorption means C (FIG. 4b), the photo detector 6 andthe signal processing circuit 7 are located in a housing 30. Thesleeve-like housing 30 has at its front side 30a a coupling means 31 bywhich the housing can be connected to a light source which is not showneither directly or by means of a correspondingly formed fiber optic. Alight beam outlet and inlet aperture 32 is provided at the oppositefront side 30b. This is joined by a peg-like tube 33 which supportsholding means 34 for the convex lens 4 displaceably in the direction ofthe arrow 35 or vice versa. Preferably, the housing 30 is closed inorder to protect the relatively sensible measuring optic 3, 8b, C, 6 and7 from dirt and damage.

In FIG. 5 the Doppler frequency for a selected web is shown in itsrelation to the velocity of the web. In this graph typical oscillogramsare shown for certain velocities of the measured web wherein the Dopplerfrequency f is correspondingly increased with increasing web velocity.In connection with the measurement according to FIG. 5 an irradiationangle of 86.5° was selected.

FIG. 6 shows the quantum yield expressed in percentage in relation tothe wavelength of the light beams for two typical photo diodes whereinthe lower curve (up to 900 nm) corresponds to a p-i-n-type diode and theupper curve (up to 900 nm) corresponds to a silicone avalanche photodiode. As a comparison of the oscillograms also shown in FIG. 6 whichhave been made at a wavelength of the light beam of 632.8 nm and at acapacity of 3 mW shows, the resolution of the avalanche diode is betterthan that of the p-i-n-type diode at the above-cited wavelength. This ischanged only at a wavelength of about 900 nm at which the p-i-n-typediode allows a higher quantum yield than the avalanche diode. From thisit can be taken that the photo diode used has to be tuned to thewavelength of the light beam.

In FIG. 7 a further embodiment of the apparatus is schematically shownwhich allows an exact control of the velocities of two webs. Here, afirst web 20 is transported in the direction of the arrow 22 by a drivemotor 21 which can be optionally adjusted in its speed. The web 20 isassociated with a first apparatus 23 which has a construction asdescribed in connection with FIG. 1. A second web 24 is transported inthe direction of the arrow 22 by means of a second drive motor 25. Thesecond web 24 is associated with a second apparatus 26 which correspondsin its construction to the apparatus 23. As a difference with regard tothe embodiment shown in FIG. 1, the apparatuses 23 and 26 do not haveeach a separate oscilloscope. A single oscilloscope 27 is ratherprovided which has two inputs which are designated in FIG. 7 withchannel 1 and channel 2. Here, the apparatus 23 is connected to channel1 and the apparatus 26 is connected to channel 2.

The above-described apparatus functions in the following manner:

The oscillogram recorded by the oscilloscope 27 by means of channel 1and the apparatus 23 which, as shown in FIG. 2, has a certain form inresponse to the velocity of the web 20 is selected as a predeterminedvalue, for example. Thereafter, after switching to channel 2 thevelocity of the web 24 is shown on the oscilloscope in the form of anoscillogram by means of this channel and the apparatus 26. Also in thiscase this oscillogram which is associated with the velocity of the web24 has a certain form. If an exact synchronism of the two webs 20 and 24with respect to their velocities is desired, the velocity of the web 24is varied until the oscillogram received by means of channel 2corresponds in its form to the oscillogram shown by means of channel 1and selected as a predetermined value. This is achieved by the featurethat the drive motor 25 is a motor with variable speed and can bemanually adapted to the velocity of the motor 21. Furthermore, there isthe possiblity to compare the Doppler frequency f_(d1) measured by meansof channel 1 with the Doppler frequency f_(d2) measured by means ofchannel 2 wherein the following equation is true with equal velocitiesof the web 20 and 24:

    f.sub.d1 =f.sub.d2.

Accordingly, if the velocities of the webs are different the twoabove-cited Doppler frequencies are also different.

Of course, it is also possible to tune more than only two webvelocities. For this, a corresponding number of apparatuses 23 or 26 aswell as a correspondingly designed signal processing circuit 27 arenecessary.

The above-described apparatus can be used for instance in textilemachines according to which two webs are simultaneously processed. Thisis also true for example for mercerizing machines or cylindrical driersso that two webs can be mercerized or dried one upon the other insynchronism or with different exactly determined velocities.Furthermore, it is possible to use the above-described apparatus with aweb at different locations so that by this drive motors of a singlemachine or of successive machines are controlled and that the webtension significantly influencing the quality can be adjusted in anexact and reproducible manner which is not possible with the knownmeasuring method on account of the above-described faults.

In addition to the above-described manual adjustment of the speed of thedrive motor 25 it is also possible to design the signal processingcircuit 27 such that it generates an output signal which is used for thecontrol of the drive motor 25 if the Doppler frequency is different froma predetermined and/or measured Doppler frequency.

What is claimed is:
 1. A method of measuring at least one of thevelocity and length of an endless web of textile goods comprising thesteps of:generating a beam of laser light having energy density ofbetween 4 mW/cm² and 15 mW/cm² ; forming a reference light beam and aweb irradiation light beam from the beam of laser light; irradiating theweb of textile goods with the irradiation light beam; obtaining thelight reflected from the web of textile goods and forming same into areflected light beam; reducing the intensity of the reference light beamin accordance with the intensity of the reflected light beam by applyingthe reference light beam to a reflection means that reflects a portionof the applied light; applying, in superimposition, the reducedintensity reference light beam and the reflected light beam to a commonphoto detector; and determining from the frequency characteristics ofthe photo detector output at least one of the velocity and length of theweb of textile goods.
 2. The method according to claim 1 further definedas directing the beam of laser light to the web at an irradiation angle(α), relative to the running direction of the web, of between about 20°and 89.5° or between about 90.5° and about 160°.
 3. The method accordingto claim 2 further defined as directing the beam of laser light to theweb at an irradiation angle (α) of between about 80° and about 89.5° orbetween about 90.5° and about 100°.
 4. The method according to claim 3further defined as directing the beam of laser light to the web at anirradiation angle (α) of 86.5° or 93.5°.
 5. An apparatus for measuringat least one of the velocity and length of an endless web of textilegoods comprising:a light source for generating a beam of laser lighthaving an energy density of between 4 mW/cm² and 15 mW/cm² ; beamdividing means for forming, from said beam of laser light, a referencelight beam and an irradiation light beam for irradiating the web oftextile goods; means for forming the light reflected from the web oftextile goods into a reflected light beam; means for reducing theintensity of the reference light beam in accordance with the intensityof the reflected light beam, said means comprising reflection meansreflecting a portion of the light applied thereto; and evaluation meansincluding a single photo detector means for receiving the reducedintensity reference light beam and the reflected light beam insuperimposition, said evaluation means determining the frequencycharacteristics of the superimposed light beams for providing, from saidfrequecy characteristics, an indication of at least one of the velocityand length of the web of textile material.
 6. The apparatus according toclaim 5, characterized in that the laser is an inert gas laser.
 7. Theapparatus according to claim 6, characterized in that the laser is aHe-Ne-laser.
 8. The apparatus according to claim 5 characterized in thatsaid photo detector has a measuring area for receiving the reference andreflected light beams of <1 mm².
 9. The apparatus according to claim 8characterized in that said photo detector has a measuring area of about0.2 mm² to about 0.5 mm².
 10. The apparatus according to claim 5characterized in that said photo detector is an avalanche photodiode.11. The apparatus according to claim 10 characterized in that said photodetector is a silicon avalanche photodiode.
 12. The apparatus accordingto claim 5 characterized in that the photo detector is a barrier layerphoto detector having a high, frequency resolution in the range of about500 Hz up to about 1000 kHz.
 13. The apparatus according to claim 12characterized in that the barrier layer photo detector has a high,frequency resolution in the range of about 500 Hz.
 14. The apparatusaccording to claim 5 characterized in that said means for forming thelight reflected from the web into a reflected light beam includes aconvex lens.
 15. The apparatus according to claim 14 characterized inthat said convex lens has a focal length of between about 40 mm andabout 60 mm.
 16. The apparatus according to claim 15 characterized inthat said convex lens has a focal length of about 50 mm.
 17. Theapparatus according to claim 14 characterized in that said convex lenshas a diameter of less than 25 mm.
 18. The apparatus according to claim17 characterized in that said convex lens has a diameter of about 19 mm.19. The apparatus according to claim 14 wherein said convex lens is sopositioned with respect to said beam dividing means that the irradiationlight beam passes through said convex lens.
 20. The apparatus accordingto claim 5 wherein said reflection means includes light absorptionmeans.
 21. The apparatus according to claim 5 wherein said evaluationmeans includes an oscilloscope coupled to said photo detector fordisplaying the frequency characteristics of the superimposed light beamsas an oscillogram.
 22. The apparatus according to claim 5 furtherincluding an elongated housing having a pair of opposed ends, saidhousing having means for coupling said housing to said light source atone of said ends, said other end having aperture means emitting theirradiation light beam and having said forming means for said reflectedlight beam; said beam dividing means, and at least a portion of saidevaluation means being located within said housing.
 23. The apparatusaccording to claim 14 further including an elongated housing having apair of opposed ends, said housing having means for coupling saidhousing to said light source at one of said ends, said other end havingaperture means emitting the irradiation beam; said beam dividing means,and at least a portion of said evaluation means being located withinsaid housing; said aperture means including said convex lens and meansfor moving said lens relative to said housing toward and away from theweb of textile goods.
 24. The apparatus according to claim 5 whereinsaid light source generates said beam of laser light along a line ofgeneration and wherein said reflection means comprises a mirror systeminterposed between said beam dividing means and said evaluation means,said mirror system reflecting said deflected light beam along a pathhaving a plurality of segments, at least one of which lies parallel tothe line of generation of the beam of laser light.
 25. The apparatusaccording to claim 5 further defined as one for measuring the velocityof a web at a plurality of locations or measuring the velocity of aplurality of webs, said apparatus comprising at least one light source,a plurality of beam dividing means for irradiating a web at a pluralityof locations or irradiating a plurality of webs, and a plurality ofintensity reducing means, and wherein said evaluating means has a singlephoto detector for each pair of reference and reflected light beams forproviding an indication of the velocities of a web at a plurality oflocations or of a plurality of webs.
 26. The apparatus according toclaim 25 wherein said evaluation means includes an oscilloscope coupledto said photo detectors.
 27. The apparatus according to claim 25 furtherdefined as one for controlling the velocity of a web at at least a pairof locations or controlling the velocity of at least a pair of webs, thevelocities of the web or webs being established by a plurality of motivepower means, and wherein said evaluating means provides output signalsto said motive power means for controlling the velocities of said web orwebs.